The present invention relates to fabric dewatering devices, and more specifically to the dewatering of continuously moving fabrics used in papermaking machines, wherein the fabric is guided through a small-radius turn which causes water to be expelled from the fabric via centrifugal force.
Permeable fabrics, or belts, are often used in paper machines for supporting a paper web during the papermaking process. After the web is separated from the fabric, the fabric typically undergoes a partial or full-width cleaning or washing process. During the cleaning process, the fabric is exposed to water which the fabric tends to retain when it returns to receive a new portion of the paper web. Papermaking applications that involve a process of thermal drying of the paper web supported on a drying fabric are sensitive to the quantity of residual water retained by the fabric, since retained water may rewet the paper web. Thus, such drying fabrics must be treated to remove all, or at least a major part of, the residual water after cleaning. Other applications may also require fabrics to be at least partially dewatered to prevent the water from slinging onto other parts of the machinery or other parts of the paper web while the fabric is returning from the cleaning process to receive a new portion of the paper web.
Tissue paper production requires special types of fabrics to achieve a final product with a high bulk. Very often TAD fabrics, or other types of tissue-making fabrics or belts, are used for the manufacture of textured tissue or web. This requires a special textured structure of the fabric itself and, consequently, a definite fabric thickness. Thick, structured fabrics are especially prone to water absorption during washing and retention of that water in the deeper parts of the fabric structure.
Pressing is a conventional means of dewatering fabrics that tends to be effective for non-woven fabrics such as felt. However, pressing is not as effective for simple woven fabrics such as those used for forming or TAD applications. Such woven fabrics are prone to retain water due to their thickness and less compressible structure.
A roll press for squeezing water from a papermaking felt is disclosed in Great Britain Patent No. 1,273,827 (""827). The press has two press rolls with parallel axes and an intermediate roller which is of substantially smaller diameter than that of either press roll. The intermediate roll is located between the two press rolls so as to form two press nips with the respective press rolls. The intermediate roll is arranged offset to one side of the common axial plane of the two press rolls and is movable toward this common axial plane. The felt runs into the first nip from the side of the common axial plane remote from the intermediate roll and leaves the second nip towards the remote side. Tension in the felt draws the intermediate roll against the press rolls with sufficient linear pressure to compress the felt so as to squeeze water from the felt.
Vacuum pans are well known in the art for dewatering fabrics and consist of a collection pan connected to a vacuum source and in proximity to the travelling fabric. The vacuum source exerts a suction pressure on the fabric, drawing water out of the fabric and into the pan. Another well-known method is to use an air knife that blows air out a narrow slot and through the fabric, thus blowing water out of the fabric and into a collection pan. U.S. Pat. No. 4,116,762 to Gardiner (""762) teaches the use of a hollow, foraminous cylinder over which a felt is passed. The cylinder allows air flow through to the travelling felt to drive water out of the felt. For fabrics that are very permeable, methods such as the ones described above involving blowing or sucking air through the fabric require a very large air flow and flow velocity, and hence consume a great deal of energy.
Centrifugal force has been used to aid in the dewatering of fabrics by running the felt over a curved surface with a small radius at high speeds. The ""827 and ""762 patents use centrifugal force to aid dewatering to a certain extent. As another example, U.S. Pat. No. 6,153,056 to Schiel (""056) discloses a draining device that drains water by circulating a press felt loop about a short region of convex curvature on a guide roll. The centrifugal force displaces water out of the belt and into a collecting device.
Whenever a wet moving fabric changes direction, by passing around a roll or foil for instance, there is a tendency to throw off water. The magnitude of this tendency depends both on the angular velocity and duration for which this is maintained. A small radius and a large wrap angle will tend to maximize the water removal tendency. In the case of a lead roll, a small radius is difficult to achieve, especially when combined with a large wrap due to problems with roll deflection and critical speeds. Therefore, in the case of a lead roll a small radius is not practical, although a large wrap presents no difficulty. In the case of a foil or stationary element, a small radius can readily be achieved but the wrap must be severely limited in order to avoid fabric wear.
The present invention meets these and other needs, and is characterized by a fabric dewatering device and method in which a fabric to be dewatered is passed over a leading guide roll and a trailing guide roll. The leading guide roll is rotatable about its axis and the trailing guide roll is rotatable about its axis and is parallel to the axis of the leading guide roll. The leading and trailing guide rolls are spaced apart such that the fabric passes over a portion of a circumference of the leading guide roll and then over a portion of the circumference of the trailing guide roll, in the same rotational direction about both rolls. The fabric is wrapped about both rolls so as to form a fabric loop between the leading guide roll and the trailing guide roll. This fabric loop includes a trough portion spaced to one side of the plane defined by the axes of the guide rolls.
A control device controls passage of the fabric through the fabric dewatering device so as to maintain the position of the trough portion of the fabric loop. In one embodiment, the control device includes a drive connected to each of the rolls and operable to rotate each roll about an axis thereof in the same rotation direction. A sensor is used for detecting a position of the trough portion of the fabric loop and is connected to a controller. The controller and the drive control the rotational speed for each roll so as to maintain the position of the trough portion spaced to one side of the axes of the rolls such that the fabric loop has a radius of curvature sufficiently small to cause water to be expelled from the fabric by centrifugal force.
In another embodiment the control device includes a rider roll inserted within the trough portion so as to maintain the position and geometry of the fabric loop. The rider roll preferably has a diameter in the range of 50 mm to 100 mm. Smaller diameters result in greater centrifugal forces, but decrease the dwell time while larger diameters increase the dwell time of the fabric. Advantageously, the fabric has a wrap angle around the rider roll of about 200xc2x0 to 300xc2x0. Tension in the fabric draws the rider roll against the guide rolls but there is no attempt to compress the fabric to squeeze water from the fabric. Rather, dewatering is accomplished primarily by centrifugal forces on the fabric passing about the rider roll. Dewatering can also be aided, in some embodiments, by making the rider roll permeable, and forcing or drawing air through the rider roll and the fabric wrapped thereabout.
In one embodiment, maintenance of the trough portion below the axes of the guide rolls is facilitated by way of a lead nip and a trailing nip. The lead nip is formed between a first surface and the leading guide roll while the trailing nip is formed between a second surface and the trailing guide roll. The fabric passes through the lead nip upstream of the trough portion and then through the trailing nip downstream of the trough portion. The first and second surfaces can be portions of a single top roll or two separate top rolls. Tension in the fabric is relieved in the lead nip such that the fabric loop is essentially free of tension in the machine direction.
In another embodiment, the two nips are formed by a single top roll that is deformable. The top roll is pressed with a greater force against the trailing guide roll than against the leading guide roll, whereby the trailing nip has a greater indentation than the lead nip. Any of the three rolls can be driven. The loop length can be regulated by controlling the indentation of the trailing nip.